This invention relates to speech processing systems, and more particularly, to pitch detectors.
The pitch of a human voice is that component of speech produced by the vibration set up in the vocal cords of the speaker. In the speech process the vibrations of the vocal cords are convolved with resonant frequencies in the head of the speaker, known as formants, to produce a majority of the sounds heard in human speech. However not all of the common English sounds are produced in this manner. Some sounds do not involve the use of the vocal cords as for example sounds produced by bursts of sound, and sounds made by setting up turbulances inside of the head of the speaker.
Although the pitch is not present in every sound made by a human, it is highly desirable to be able to recover the pitch information for such functions as speech compression for transmission of analog speech with narrow bandwidths, and also for speech recognition by electronic means. Although the basic concept of pitch is readily understood, the subtleties of pitch are complex. See for example Schroeder, "Models of Hearing," 63 Proceedings of the IEEE 1332 (1975). Pitch detection has been a difficult problem in speech since the pitch range of many speakers is greater than 1 octave, and the pitch range of the population is 3 octaves wide. In general the pitch of a speaker is always at a lower frequency than the lowest formant frequency. However, in vowels of high first formant frequency, or in radio channels with low frequency cutoff above 100 hertz, the fundamental harmonic of the pitch frequency may be very low in amplitude compared with the energy of other harmonics in the speech waves. In general, linear filtering is not usually adequate to recover only the pitch fundamental. Computer programs have been developed using convolution techniques which are 90 to 95 percent successful in recovering pitch information. However these systems do not operate in real time, and involve the use of fairly extensive hardware.
Therefore it can be appreciated that a pitch detector which operates in real time and involves fairly inexpensive, commonly found electronic circuit elements is highly desirable.